AbstractYield of tea plant (Camellia sinensis L) is reduced in old farms due to the compactness of the soil around the roots. As a consequence, water flow and air exchanges will be reduced. Operating in the thin layer of soil where plant roots develop, earthworms improve recirculation of the nutrients and microbial metabolites and stimulate nutrient uptake. Evaluation of the biological effects of the earthworm on the yield of tea was the main objective of this study. Experiment was conducted on the basis of a randomized complete blocks design with four replications in two years. Treatments were earthworm, chemical fertilizer (NPK), cow manure and control (no treatment). Wet weight of the leaves was measured each year. Plots with earthworm and chemical fertilizer significantly yielded higher than those with cow manure and control. Based on the results of this study, biological influence of the earthworm was confirmed to improve yield of tea plants.